The present invention relates to methods and apparatus for controlling acceleration and velocity of a stepper motor and more particularly to such methods and apparatus in which the stepper motor switching angle varies.
Stepper motors transfers with high accuracy digital electrical signals directly into discreet angle movements (steps) of a rotor. Stepper motors are synchronous motors in which rotor's positions depends directly on driving signal. Rotary moment is defined by magnetic energy and is proportional to the tooth number of the rotor.
Most stepper motors are operated in an open-loop configuration. Given a known and fixed load driven by the stepper motor, a commutation (step) sequence can be developed by a person having ordinary skill in the art which accelerates the load to a desired velocity without a loss of steps. In other words, each time the motor is commutated, the rotor is in a position in which torque generated by the electrical field advances the rotor until it is appropriately positioned for the next commutation, and so forth. In such open-loop configurations, if the load is different than that for which the commutation sequence was developed, steps can be lost to the extent that the rotor may not rotate at all.
Closed-loop configurations use feedback to sense rotor position via a conventional shaft encoder. The rotor position information may be utilized to produce each motor commutation. Most such configurations employ a fixed switching angle commutation. In other words, the motor is commutated each time the rotor advances through a predetermined angle. While the motor does not lose steps with this method, it cannot achieve a precise target velocity due to uncontrollable variance in parameters such as motor supply voltage, friction, etc. Also, if the load is different than that for which the commutation switching-angle was chosen, the stepper motor may run at a velocity very different from the desired velocity. Moreover, when a desired velocity is selected, the corresponding switching angle typically cannot be calculated with precision.
One more technique is driving the motor by supplying currents that are sequentially phase shifted. In this case, the rotary moment is less oscillating.
The rate with which current changes significantly affects dynamic characteristics of the motor. Different techniques has been known that provides the increase in current build-up in motor coils, among which are:                introduction of the additional resistance which reduces the time constant of current. The increase of the maximal current in a loop is compensated by the increase in the driving voltage. The drawback of this technique is electrical power loss in the resistor;        electrical damping by spooling an additional coil together with the main coil. In this case, the additional coil is coupled with the main one and its time constant can be controlled by an external resistance. Such closed-loop winding acts only during the transitional periods thus reducing substantially time constant. The drawbacks of the method is power reduction and increase in motor' moment of inertia;        high-voltage feeding of a winding until the required current appears. The drawback is increase in power loss and the complexity of driving circuitry.        
The foregoing prior art stepper motor systems will not accurately control the velocity and position of a load during the acceleration and deceleration periods and experience substantial power losses and temperature increase which imposes limitations on operation characteristics of the motor.